1. Field of the Invention
The present invention relates to orthogonal frequency division multiplexing (OFDM), and more particularly, to a method of determining a training signal so as to facilitate acquisition of symbol sync, frequency offset estimation, and channel estimation in an OFDM system, and an apparatus and method for receiving a baseband OFDM signal using the training signal.
2. Description of the Related Art
Generally, in an OFDM signal, symbols in parallel are transmitted in series, so symbol sync is necessary for an OFDM receiver to acquire the start of an OFDM symbol in order to convert a transmitted serial signal to symbols in parallel. In addition, in an OFDM mode, interchannel interference is caused by a frequency offset between a received carrier frequency and an oscillator frequency of the OFDM receiver, so the OFDM receiver requires a frequency offset estimator for accurately estimating a frequency offset. The OFDM receiver also requires a channel estimator for estimating the gain of a channel.
Among many elements of such an OFDM receiver, the frequency offset estimator most significantly influences the performance and complexity of the OFDM receiver. In the meantime, conventional frequency offset estimation based on an autocorrelation function has an advantage of a small amount of computation but has a disadvantage of a narrow acquisition range for a frequency offset. Conventional frequency offset estimation based on a cross-correlation function has an advantage of no limitation to an acquisition range for a frequency offset but requires to calculate correlation values at each subcarrier, and thus the OFDM receiver is complicated due to a large amount of computation.
In order to allow an OFDM receiver to acquire the start of an OFDM symbol and accurately synchronize a carrier frequency with an oscillator frequency during demodulation of a received signal, an OFDM transmitter transmits a predetermined training signal to the OFDM receiver.
FIG. 1 shows the structure of a conventional training signal. As shown in FIG. 1, an OFDM transmitter using N subcarriers allocates data A1 through AN to the N subcarriers, respectively, in a frequency domain during a first symbol period, thereby structuring a first training symbol shown in FIG. 1(a), and allocates data B1 through BN to the N subcarriers, respectively, during a second symbol period, thereby structuring a second training symbol shown in FIG. 1(b). The data A1 through AN of the first training symbol are related with the data B1 through BN of the second training symbol by Formula (1).Bk=AkCk, k=1˜N  (1)
More accurately, N indicates the size of inverse fast Fourier transform (IFFT), which implements an OFDM modulator, and since a guard band, in which a subcarrier is not transmitted, is set at the edge of a transmission band in order to facilitate filtering and avoid interference from a signal in an adjacent band, the number of actually used subcarriers should be expressed by L≦N. However, hereinafter, for clarity of the description, it is assumed that L=N, and thus the number of subcarriers is denoted by N.
Accordingly, the OFDM transmitter repeatedly transmits the same data over the same subcarriers during the two symbol periods when Ck≠1 or transmits the first data and the second data particularly related with the first data over the same subcarriers during the two symbol periods when Ck≠1. Thereafter, the OFDM transmitter transmits data X1 through XN of a normal data symbol shown in FIG. 1(c) over the respective subcarriers. Hereinafter, a set of the first training symbol and the second training symbol is referred to as a “training signal”.
A conventional OFDM receiver recovers symbol timing using the first and second training symbols and estimates a frequency offset. As shown in FIG. 2, the conventional OFDM receiver includes an analog-to-digital (A/D) converter 110, which converts a baseband analog signal to a digital signal; a symbol timing recovery unit 120, which detects the start of an OFDM symbol; a guard interval eliminator 130, which eliminates a guard interval from the OFDM symbol using the recovered symbol timing; a fast Fourier transform (FFT) unit 140; a fractional frequency offset estimator 150, which estimates a frequency offset smaller than subcarrier spacing; an integer frequency offset estimation unit 160, which estimates a frequency offset that is an integer multiple of the subcarrier spacing; an adder 170, which adds a fractional frequency offset and an integer frequency offset to calculate an entire frequency offset; and a multiplier 180, which configures the entire frequency offset into a complex exponential function indicating a change in a phase and multiplies the complex exponential function by the output of the A/D converter 110.
The integer frequency offset estimation unit 160 includes a symbol delay block 161, a conjugate complex number calculator 162, a correlation function calculator 163 calculating a correlation function using a delayed received symbol and a received symbol not delayed, and an integer frequency offset estimator 164.
In operation of the conventional OFDM receiver shown in FIG. 2, when an OFDM transmitter transmits a training signal, which is configured as shown in FIG. 1, periodically or transmits the training signal by attaching it to the front of a data symbol when signal transmission is newly started, the OFDM receiver receives a signal transmitted from the OFDM transmitter and demodulates the signal into a baseband analog signal. Thereafter, the A/D converter 110 converts the analog signal to a digital signal. The symbol timing recovery unit 120 detects the start of a symbol from the digital signal in order to acquire symbol sync. The guard interval eliminator 130 eliminates a guard interval in a front portion of an OFDM symbol using the detected start of the symbol. The FFT unit 140 performs FFT on the OFDM symbol from which the guard interval has been eliminated.
Simultaneously with the acquisition of symbol sync by the symbol timing recovery unit 120, the fractional frequency offset estimator 150 estimates a frequency offset Δf1 less than subcarrier spacing in the digital signal. The repetitive characteristic of the training signal is used for the symbol sync acquisition of the symbol timing recovery unit 120 and the fractional frequency offset estimation of the fractional frequency offset estimator 150.
The integer frequency offset estimation unit 160 receives a signal output from the FFT unit 140 and estimates a frequency offset that is an integer multiple of the subcarrier spacing. More specifically, the symbol delay block 161 delays a first training symbol output from the FFT unit 140 by a unit symbol period. The conjugate complex number calculator 162 calculates a conjugate of the first training symbol. The correlation function calculator 163 receives a signal output from the conjugate complex number calculator 162, i.e., the conjugate of the first training symbol, simultaneously with a second training symbol output from the FFT unit 140 and calculates a correlation function with respect to the second training symbol and the conjugate of the first training symbol in units of data samples. The integer frequency offset estimator 164 obtains a position, at which a correlation value is maximum, from the correlation function and estimates an integer frequency offset Δf2, which is an integer multiple of the subcarrier spacing.
The adder 170 adds the frequency offsets respectively output from the fractional frequency offset estimator 150 and the integer frequency offset estimation unit 160. Thereafter, the multiplier 180 configures the entire frequency offset Δf1+Δf2 into a complex exponential function e−j2π(Δf1+Δf2)n/N and multiplies the complex exponential function by the digital signal output from the A/D converter 110 so that a frequency offset occurring between a carrier frequency and a frequency of an oscillator in the OFDM receiver during demodulation can be corrected.
In addition, the conventional OFDM receiver estimates an integer frequency offset and simultaneously estimates channel gain in the frequency domain. Then, conventional OFDM receiver obtains accurate channel gain with the cyclic shifting of a subchannel as many times as the estimated integer frequency offset.
As described above, in order to estimate an integer frequency offset between an oscillator frequency and a carrier frequency, the conventional OFDM receiver calculates correlation values by performing correlation between sequential two OFDM training symbols in units of data samples and obtains a frequency shift that gives the maximum correlation value. Such conventional technology has the following problems.
Firstly, in the case of an OFDM system using N subcarriers, when an integer frequency offset is estimated, a maximum of N−1 correlation values must be calculated in order to calculate spacing giving the maximum correlation value. In this case, the amount of calculation is great, so it takes a long period of time to acquire sync in an OFDM receiver.
Secondly, since correlation values must be calculated with respect to samples one by one in order to obtain a correlation function, the circuit of the OFDM receiver is complex.
Due to a great amount of calculation and high complexity, the conventional OFDM receiver is not suitable to mobile communication systems using portable terminals.